1. Field of the Invention
This invention relates to a magneto-striction type stress detector being high in stress detecting sensitivity and satisfactory in linearity.
2. Description of the Prior Art
There has heretofore been used a method of detecting stress wherein a value of strain generated in a ferromagnetic material due to a stress is detected by use of a magneto-striction effect. According to this test method, as a body to be measured, there are used a test piece made of the ferromagnetic material itself or a test piece of a body to be measured which is attached thereto with a ferromagnetic material. A value of strain is measured by a sensor in which an exciting coil for alternatingly magnetizing this magnetic material in one direction is combined with a coil for detecting a component of a magnetic flux perpendicularly intersecting the exciting direction in the magnetic material.
It is known that a detection signal by this detecting coil includes a variation in amplitude component and a variation in phase component, and a ratio between the both components is varied and not constant with the increase or decrease of a stress. Now, since, in the conventional detector for processing a detection signal by an envelope detection method or a square detection method, only a variation in amplitude component of the detection signal is detected, such disadvantages are presented that the linearity is deteriorated and the sensitivity is lowered. Particularly, even if it is intended to improve the accuracy in the geometric rectangular positional relationship of the both coils to thereby raise the detecting sensitivity, the phase component is neglected. Therefore, such a disadvantage is presented that, on the contrary to the intention, the sensitivity is lowered.
Description will hereunder be given of a conventional example of torque detection from the detection of a torsional stress generated in a shaft with reference to the drawings. The conventional example is shown in FIG. 2. FIG. 3 is a sectional view taken along the line III--III in FIG. 2. A torque sensor 1 of the conventional type includes a U-shaped exciting core 11 disposed in parallel to the axis of a shaft 2 to be measured and a detecting core 12 perpendicularly intersecting the exciting core 11 and magnetically separated from the exciting core 11. The exciting core 11 is wound therearound with an exciting coil 111 and the detecting core 12 is wound therearound with a detecting core 121. The shaft 2 to be measured itself is constituted by a ferromagnetic material.
The detecting sensor 1 is adapted to detect a stress generated in the shaft 2 to be measured through a detection circuit shown in FIG. 4. Here, an AC (alternating current) power source 31 is adapted to generate a sine wave voltage, an output from the AC power source 31 is applied to the exciting coil 111 and alternatingly magnetizes the shaft 2 to be measured. When a stress is generated in the shaft 2 to be measured, a magnetic flux is generated in a direction perpendicularly intersecting the exciting direction under a magneto-striction effect, and a magnitude of the magnetic flux is detected by the detecting coil 121 as an induced voltage. A signal by this induced voltage is amplified in an AC amplifier 32, thereafter, subjected to an envelope wave detection for example, in a wave rectifier 33, and an amplitude component is outputted. A signal wave form at a point A and an output voltage at a point B in FIG. 4 are shown in FIGS. 5 and 6, respectively. In addition, a graphic chart in FIG. 5 shows a change in wave form of an AC signal with an applied torque at the point A, indicating the conditions of the amplitude and the phase simultaneously. Wave form curves x1, x2 and x3 indicate that applied torques are -5 kgm torque to the left (inverse direction), zero torque (0 Kgm) and 5 kgm torque to the right, respectively. The frequency of the wave form is equal to the frequency of an exciting voltage applied to the exciting coil 111, i.e. the alternating voltage of the AC power source 31. Furthermore, in the graphic chart of FIG. 6, an applied torque value to the right or left is given as an abscissa and an output voltage at the point B is given as an ordinate. As the applied torque varies from the left to the right, the output voltage increases, whereby an applied torque value to the shaft to be measured from this output voltage value is found.
However, as apparent from FIG. 5, the wave form of the AC signal is varied in both amplitude and phase due to an applied torque, and the interaction therebetween depends upon the applied torque, whereby, as apparent from FIG. 6, the relationship between the applied torque and the output voltage is of non-linearity, whereby it is difficult to accurately seek the applied torque only from the output voltage, and, in order to accurately measure, it is necessary to use a complicated linearizing circuit or a linear calculation.
The following reasons lead to the above-described phenomenon.
Notwithstanding, here, description will be given with a simple model of a cosine wave. This model is easily realized with a filter circuit and the like, and, even with a complicated wave form such as a triangular wave, if Fourier analysis is applied thereto, explanation can be made similarly.
If a component of the detection signal depending upon the torque from the detecting sensor is f.sub.1, then f.sub.1 can be represented by the following equation. EQU f.sub.1 =A.sub.1 (T) cos (.omega..tau.+.phi..sub.1) (1)
where
A.sub.1 (T) is an amplitude dependent upon a torque, PA1 .omega. an angular velocity, .tau. a time, and PA1 .phi. a phase.
In practice, simultaneously with f.sub.1, there is a component f.sub.2 of the detection signal not dependent upon the torque, which is represented by the following equation EQU f.sub.2 =A.sub.2 cos (.omega..tau.+.phi..sub.2) (2)
where A.sub.2 is the amplitude not dependent upon the torque.
A detection signal f.sub.0 is represented by the following equation. ##EQU1## provided that EQU A.sub.0 (T)={A.sub.1 (T).sup.2 +A.sub.2.sup.2 +2A.sub.1 (T)A.sub.2 cos (.phi..sub.1 -.phi..sub.2)}.sup.1/2 ( 4) ##EQU2##
In consequence, in general, both the amplitude A.sub.0 (T) and the phase .phi..sub.0 (T) are complicated functions of the torque (T), and consequently, each of the amplitude A.sub.0 (T) and the phase .phi..sub.0 (T) is not in proportion to the torque (T).
However, when a condition of that cos (.phi..sub.1 -.phi..sub.2)=.+-.1 is established, the above-mentioned equations (4) and (5) are developed to the following equations EQU A.sub.0 (T)=.vertline.A.sub.1 (T).+-.A.sub.2 .vertline.(the reference numerals are arranged in the order of the present condition) (6) EQU .phi..sub.0 (T)=Co(.phi..sub.1 is a reference basis; Co is a constant not dependent upon the torque) (7)
Thus, a linear output is obtained only by the detection of the amplitude value, and the phase is not varied. However, in general, the condition of that cos (.phi..sub.1 -.phi..sub.2)=.+-.1 is not established.